Dancing in the dark

“Work like you don’t need the money. Love like you’ve never been hurt. Dance like nobody’s watching.”–Satchel Paige

To the silent tune of gravity, congeries of celestial objects – big and small – dance each night away. In the darkness beyond Neptune, this troupe of Kuiper Belt objects (KBOs) had been dancing like no one was watching – until now.

Their dance is a slow one, for Kuiper Belt objects take centuries to complete one orbit. These KBOs, each a few hundred kilometers in size, have been discovered by DES over the last two and a half years. (One of them was described here earlier.) Suppose you knew nothing about gravity. What would you make of a pattern like this? How would you explain it? The laws that give rise to such intricate celestial swirls must be incredibly complicated, right?

Ancient people marked the wanderings of the planets from night to night and season to season. They noticed that they moved across the sky at wildly different rates: sometimes, they appeared to stop, turn around, and move backwards against the canopy of fixed stars, before turning again and resuming their course. Ingenious models were developed to explain this complicated dance. But they became increasingly unwieldy, and even worse, failed to describe new and more accurate observations.

It took two scientific revolutions—first from Copernicus and then from Newton—to show that planetary motion could be readily explained by a single simple equation, the law of gravitation. The hidden pattern suddenly became clear.

The graceful pirouette executed by a KBOs arises from a combination of two motions. Its centuries-long orbit produces a slow eastward drift that carries it about the width of one DECam field of view per year. But we observe these objects from a moving platform, planet Earth. As the earth makes its journey around the sun, we observe the KBO from different perspectives, sometimes from 150 million kilometers on one side of the sun, six months later from 150 million kilometers on the other, and at other times from somewhere in between. This results in an annual back-and-forth motion relative to the distant stars that’s superimposed on the KBO’s own orbital motion. Watch how your fingertip moves against background objects when you move your head from side to side and you’ll get the idea.

Physics aims to distill order from complexity, to explain the vast array of natural phenomena with a small number of simple laws. Eventually, physicists learned that Newton’s law of gravitation fell short in certain situations and needed to be superceded by Einstein’s theory of general relativity.

Today, gravity confronts our generation with a new puzzle on the grandest of scales: Why is the expansion of the universe accelerating? Perhaps some new law will explain the mystery of dark energy with the as much elegance and simplicity as the dance of the planets. That’s the hope that keeps our dark energy detectives patiently looking up.